The present invention relates to an A/F (air/fuel) ratio sensor used to detect the A/F ratio of the intake air/fuel mixture to an internal combustion engine or other combustor on the basis of the concentration of oxygen in the exhaust from the engine or combustor.
Various A/F ratio detectors have been used to detect the A/F ratio of the air-fuel mixture fed to an internal combustion engine or other combustor on the basis of the concentration of oxygen in the exhaust gas. One such device, as described in Unexamined Published Japanese Patent Application No. 178354/1984, includes two spaced elements, each having porous electrodes formed on opposite sides of a tubular oxygen-ion-conductive solid electrolyte. One of the elements is used as an oxygen pump for pumping oxygen out of the gap between the two elements, and the other element is used as an oxygen concentration electrochemical cell which produces a voltage in accordance with the difference in oxygen concentration between the gap and the ambient atmosphere. This device is capable of detecting the A/F ratio in the fuel-lean region of the intake mixture.
However, this type of A/F ratio sensor, which is principally designed for use only in the fuel-lean region where the exhaust gas contains a large amount of residual oxygen, produces a similar signal in the fuel-rich region where the exhaust gas contains a very small amount of residual oxygen due to the presence of oxygen-containing species in the exhaust gas such as CO, CO.sub.2 and H.sub.2 O. In other words, each value of the output signal from the A/F ratio sensor (except for precisely at the stoichiometric value) can represent either one of two different values of A/F ratio; that is, the sensor output is ambiguous with respect to the fuel-lean and fuel-rich regions. Therefore, this A/F ratio sensor cannot be used for A/F ratio control unless it is known definitely whether the combustor is operating in the fuel-lean or fuel-rich region.
To overcome this problem, it has been proposed to introduce atmospheric air in contact with the side of the oxygen concentration electrochemical cell which does not face the oxygen pump, thereby preventing the detected signal from being inverted in the vicinity of the stoichiometric value of the A/F ratio. However, in order to introduce atmospheric air in contact with one side of the oxygen concentration electrochemical cell, the A/F ratio sensor must be open to the ambient atmosphere, but this in turn makes the sensor construction complex due to the difficulty in making it waterproof.
In order to eliminate this problem, two modifications have been proposed. According to the first modification, instead of introducing atmospheric air into the sensor, oxygen is generated by an internal reference oxygen source device incorporating an electrochemical oxygen pump cell and provided on one side of the oxygen concentration electrochemical cell, and part of the oxygen generated in the device is caused to leak into the ambient exhaust gas through a leakage resisting portion so that the oxygen partial pressure in the internal reference oxygen source is maintained constant. (See Japanese Patent Application No. 137586/1985). This approach obviates the need for an atmospheric reference. According to the second modification, the need for an atmospheric reference is eliminated by using a "current limiting" sensor in combination with a lambda sensor. (See SAE Technical Paper No. 850379).
The above-described A/F ratio sensor where part of the oxygen evolved at the internal reference oxygen source device is caused to leak into the ambient exhaust gas has the disadvantage that if the oxygen partial pressure in the ambient exhaust gas changes, a corresponding change occurs in the oxygen partial pressure within the internal reference oxygen source. This problem is particularly pronounced when the exhaust gas is in the fuel-rich state. This is because inflamable gas components such as CO and H.sub.2 contained in an exhaust gas held in a fuel-rich condition will pass through the leakage resisting portion so that a great amount of the gas will be diffused reversely into the internal reference oxygen source. It is confirmed that the pressure of the oxygen in the source is largely reduced to generate a significant error in the output of the oxygen concentration electrochemical cell. A further problem of this sensor arises from the fact that the opening at the leakage resisting portion of the internal reference oxygen source is exposed to the exhaust gas. As a result, deposits of certain glass-forming components of the exhaust gas can form at the opening of the leakage resisting portion, leading to substantial blocking of the internal reference oxygen source and causing an abnormal increase in the internal oxygen partial pressure, which renders correct detection of the A/F ratio impossible or may even lead to complete failure of the sensor.